Method to use an optical fibre as a sensor

ABSTRACT

The present invention relates to a method to use an optical fibre as a sensor. The method involves designing an optical fibre to have a sensor area (12) where the external influence on the fibre that is to be measured, for example pressure or magnetic field, is freely able to influence the fibre. Coherent radiation is then transmitted through the sensor area in several modes. On the receiver side the radiation is split in a detector device (14) into two beams, each including the entire interference pattern created by the transmission in the fibre. After that the beams are plane-polarized in two different directions which are conducted to a signal processing device (9,9 0 ) which detects the amplitude and/or frequency content of each beam and from this forms a signal which is a measure of their amplitude and/or frequency relation, which in its turn is a measure of the external influence in the sensor area (12).

BACKGROUND OF THE INVENTION

Fibre optical sensor technique is a field that during the last ten yearshas been predicted a very expanding market. The expansion has, however,not reached the pace that was predicted. The reason for this isprincipally the difficulty to separate the sensor information from thevarying influence from the environment, both on the sensor and on thetransmission to and from the sensors. Another reason is that thesingle-mode technique that has the highest degree of sensitivity iscomplex and demands that the components used are very accurate. Themultimode technique, on the other hand, can allow a more simple designand moreover, it uses more robust components. Thereby it is possible toreach such a level for the costs for the fibre optical sensors that theybecome economically interesting. But the multimode technique has alsonot experienced a break-through, mostly due to the lower sensitivitythat hitherto has been achieved with this technique. Conventionalmultimode technique is of cause based on principles of intensitymodulation of the light that is transmitted in the core of the multimodefibre. This technique is less sensitive than the phase-sensitivedetection that the single-mode technique is based on.

The fibre optical sensors have potentially very great advantages,primarily due to their non-galvanic construction, that result in forinstance insensibility for EMP, EMC etc. The sensitivity that can beachieved is also as good as or better than what can be achieved withother types of sensors. Another interesting characteristic of the fibreoptical sensor technique is the possibility to multiplex several sensorsalong the same fibre optical cable and in that way implement sensorsystems that is able to cover for instance large surveillance zones. Thesensors of the system can be dimensioned to be sensitive to severalphysical quantities such as pressure and magnetism. Great tacticaladvantages can be achieved by this. In order to achieve this goal thesensors have to be based upon such a technique that a realization of thesensor technique is robust and functionally reliable and this at areasonable cost in comparison with conventional technique.

SUMMARY OF THE INVENTION

The present invention relates to a method to use an optical fibre as asensor. The detection is phase-sensitive and can be applied to bothsingle- and multimode fibres. The invention solves the present problem.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in more detail withreference to the attached drawings where

FIG. 1 shows a laser, a fibre, in which the radiation propagates inseveral modes and the interference pattern after the fibre,

FIG. 2 shows an interference detector according to the invention,

FIG. 3 shows a first sensor system according to the invention,

FIG. 4 shows a second sensor system according to the invention,

FIG. 5 shows a third sensor system according to the invention,

FIG. 6 shows two variants of the sensor circuit in FIG. 5 and

FIG. 7 shows a fourth sensor system according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The suggested sensor system is based on detection of variations in thephase condition of the light that is transmitted in an optical fibre.This is a more sensitive method of measuring mechanical influence on theoptical fibre than measuring the variations in the effect or theintensity of the light. A mechanical influence on the optical fibre canbe translated to serve as a measuring quantity of, for instance,acoustical waves or magnet fields. It is very important that the sensorsystem is not sensitive to external influence from the environmentalparameters, that can give rise to an increased noise level for themeasuring quantity of interest. For solutions that concernesmultiplexing several fiber optical sensor, this is of particularrelevance. The solution according to the invention is based on aprinciple which among other things means that the variations in theoptical fibre due to the environment are filtered out and that onlysignificant changes caused by the measuring quantity are presented bythe sensor or sensor system. The principle is described below.

In a single-mode fibre there exists only one mode for light having awavelength above the cut off wavelength of the fibre. Light of a shorterwavelength that is transmitted in the single-mode fibre will propagatein several modes. The single-mode fibre will then act as a multimodefibre.

In a multimode fibre there exists many directions of propagation due tothe angle of incidence to the fibre that the incident "light rays" have.These directions of propagation are called modes. If coherent light, asin FIG. 1, from a laser 1 is transmitted in the fibre 2 and the emerginglight illuminates for example a white paper, a pattern of interference 3will appear on the paper. This pattern is usually called specklepattern. The pattern is caused by the constructive or destructivecombination of the different modes of the multimode fibre. This patternof interference is extremely sensitive to external influence on thefibre as the relative phase relation is changed by the influence, whichin its turn causes a movement of the pattern of interference. If thismovement can be detected a very accurate method to detect influence onthe fibre is achieved. The coherence length for the light must be longerthan the difference in distance between the highest and the lowestmodes.

One method to detect this pattern is to place a photodiode on a fixedspot for instance on an intensity maximum and watch the intensityvariations. The disadvantage with this is that the pattern fades due tothe random phase difference between emerging modes, which do not emergeuniformly during the time. Below a method to solve the problem is given.

The phase relations can change due to changes in the spectralcharacteristics of the source of light caused by for instance modulationof the laser diode, by temperature variations of the source of light, bytemperature variations along the optical fibre and by randomdistribution of the modes in the optical fibre, for instance caused bythe concatination points. Such unwanted phase variations have an uniformeffect on the radiation in all directions of polarisation, which is notthe case for the phase variations that are caused by the externalinfluence in the sensor areas.

The light emerging from the end of the fibre is therefore split by abeam splitter 4 into two beams, see FIG. 2. Each beam includes theentire interference pattern created by the transmission in the fibre. Itis suitable to choose to split the beam into two beams that are on thewhole equally strong, even though this is not necessary. The two beamsare then plane-polarized in different directions by two polarizers 5 and6. It is suitable to choose two directions that are essentiallyperpendicular to each other, even though this is not necessary. In asuitable embodiment of the invention the two emerging beams are thendetected by detectors 7 and 8 independently from each other and thedifference in amplitude is compared in a differential amplifier 9. Thesignal coming from the differential amplifier 9 is a measure of theexternal influence.

In stead of forming a difference signal, it is possible to form, inanother type of signal processing device 9⁰, other types of signals thatare a measure of the quantity one wishes to measure. Thus it is possibleto detect the difference in the quotient between the two signals or thevariations in the frequency distribution between them.

It is possible to use several sensors, 12 or 12 and 13, according to theinvention in a sensor system. Then the radiation is transmitted in shortpulses through a distribution cable 10 in the form of an optical fibre.Along the distribution cable the sensors are distributed. The radiationpulses are coupled from the distribution cable to the sensors by meansof first optical couplers, that couple a certain amount of the effect ofthe laser pulses to the sensor and let the rest pass on to othersensors. By means of other optical couplers the radiation pulses arecoupled from the sensors to one or more return cables 11 in the form ofoptical fibres, in which the radiation pulses from different sensors aretime-multiplexed. From the return cable or cables the radiation isconducted to a signal processing device 9,9⁰ that detects the amplitudeand/or frequency content of each radiation beam, and from this forms asignal that is a measure of their amplitude relation or frequencyrelation, which in its turn is a measure of the external influence inthe sensor area. The result is presented in a presentation device.

If it is possible to make sure that all influence on the cable takesplace in special sensor areas and the rest of the optical fibre, in boththe distribution cable and return cable, is completely undisturbed,which is very difficult considering the tremendous sensitivity of thephase sensitive detection, then one sensor area 12 would be enough inthe sensors. In practice this is not easy and above all, one does notwish to be forced to protect the distribution cables and return cablesfrom external influence to the extent necessary in order to reachsuccessful results. Therefore one often designs each sensor with onesensor area 12, in the form of a sensor fibre, as well as one wellprotected reference fibre 13. By designing the sensor in this way, it ispossible to take out a reference signal at the time and place of thesignal from the sensor area and by comparison with it, filter outchanges just in the sensor fibre.

In a first variant of the invention, the sensor with a sensor fibre 12and a reference fibre 13 could be made as a fibre optical ring, in whichpulses of radiation is conducted. The radiation is first conductedthrough the reference half 13, after which a part of the radiation isconducted to the detector device. The rest will travel around in thering and passes the sensor fibre 12 and thereafter the reference fibre13, after which a part of the radiation is again conducted to thedetector device etc. with lower and lower signal amplitude. The firstsignal will then be a pure reference signal.

At least two ways to implement the system is conceivable. In the firstsystem, according to FIG. 3, an interference detector 14 is placed aftereach sensor. The radiation from the detector device 14 in the twodifferent polarisation directions are connected to a common signalprocessing device 9,9⁰ via separate optical fibres 11. Finally ademultiplexing takes place in a demultiplexor 15 in order to separatethe radiation from the different sensors.

The second system according to FIG. 4 is on the whole similar to thefirst, but on the receiver side there is some difference. Thus theradiation from each sensor 12, 13 is conducted, time-multiplexed whenthere is several sensors, by one and the same optical fibre 11 to acommon interference detector 14, that is followed by a signal processingdevice 9,9⁰ and a demultiplexor 15 in the same way as in the firstsystem.

In stead of using a sensor ring as above, in other variants of theinvention one could be using two seperate adjacent optical fibresconnected in parallel, where one is a reference fibre 13 and the other asensor fibre 12. This is shown in FIG. 5. As the signal is not runningaround in the sensor, in this case one only gets one reference signaland one sensor signal. In other respects the system can be designedexactly as in the two earlier mentioned cases. Besides, in practice oneonly considers one reference signal and one sensor signal also in thecase with the sensor ring.

FIG. 6 shows variants of the sensor coupling in FIG. 5, which inprinciple gives the same situation as in this figure. Certain systemrelated advantages can, however, occur with this method.

In some applications, especially with the sensor fibre 12 and thereference fibre 13 connected in parallel as above, it could be necessaryto introduce a well defined delay in the form of a transmission line ofa determined length in order to adjust the time relation between thereference signal and the sensor signal from one sensor.

It is possible to design a system, in which interference betweendifferent modes is only localized to the sensors. If one uses asdistribution and return fibres 10, 11 single-mode fibres having a cutoff wavelength below the wavelength of the present radiation in thesystem, so that the radiation is only transmitted in one mode in thefibre, no interference between different modes occur here. Then one usesfor the sensor part together with the connections belonging to it eithera multimode fibre or a single-mode fibre having such a high cut offwavelength that the radiation in the system have a wavelength below thiscut off wavelength, so that the radiation is transmitted in severalmodes. This solution is perhaps most advantageous in the systemaccording to FIG. 3.

FIG. 7 shows what such a system could look like, with single-mode fibresoperated in single-mode in the distribution and return fibres 10, 11 anda fibre operated in multimode as sensor fibre 12.

I claim:
 1. A method to use an optical fibre as a sensor,comprising:measuring external influence on an optical fibre in a sensorarea where the external influence on the optical fibre that is to bemeasured, for example, pressure or magnetic field, is freely able toinfluence the fibre, transmitting coherent radiation through the sensorarea in several modes, splitting radiation received on a receiver sideof a detector device into two beams, each beam including an entireinterference pattern created by transmission in the fibre,plane-polarizing the two beams in two different directions which areconducted to a signal processing device which detects the amplitudeand/or frequency content of each beam, and forming a signal which is ameasure of amplitude and/or frequency relation of the beams, which is ameasure of the external influence in the sensor area.
 2. A methodaccording to claim 1, wherein the radiation is split into two beams ofthe same intensity.
 3. A method according to claim 1, wherein the beamsare plane-polarized in two perpendicular directions.
 4. A methodaccording to claim 1, wherein the radiation is transmitted in shortpulses along a distribution cable in the form of an optical fibre andcoupled by first optical couplers along the distribution cable to anumber of sensors, each comprising a sensor area, that the radiationfrom the sensors is coupled by second optical couplers to one or morereturn cables in the form of optical fibres, in which the pulses ofradiation is wavelength multiplexed.
 5. A method according to claim 1,wherein each sensor is two adjacent parts of optical fibres, one ofwhich is a reference fibre and the other a sensor fibre.
 6. A methodaccording to claim 5, wherein each sensor is designed as a fibre opticalring, in which the radiation is conducted, said ring having a referencehalf, to which the radiation is first conducted, and a sensor half, thatthe radiation is conducted to the detector device from the referencehalf, which result in the first signal to the detector device being apure reference signal, after which follows signals that have passed thesensor fibre and the reference fibre a successively increasing number oftimes.
 7. A method according to claim 5, wherein each sensor is twoseparate optical fibres connected in parallel.
 8. A method according toclaim 4, wherein a detector device is placed after each sensor, fromwhich devices the radiation in the two directions of polarisation isconducted in an optical fibre each to a common signal processing deviceand then, in a system with several sensors, to a demultiplexor.
 9. Amethod according to claim 4, wherein the radiation from the sensors isconducted in one optical fibre to a common detector device and a signalprocessing device and then, in a system with several sensors, to ademultiplexor.
 10. A method according to claim 1, wherein only thoseparts of a fibre that is part of a sensor is chosen such that theradiation is transmitted in several modes, while the other parts of thefibre is chosen such that the radiation is transmitted in only one mode.